In a flash memory it is desired to write data in the fastest manner possible. As such, multiple bytes of data are often written to the memory in one write operation. Because flash write operations are not always successful on a first memory cell program operation, data check and rewrite operations are often employed. Thus, a write operation is conducted on a group of memory cells and the state of the memory cells following the write operation is determined. If any of the memory cells have not been written to a correct logic state, a second write operation is performed on the same memory cells. Because of the necessity to rewrite memory cells, each write operation often requires multiple write cycles which adversely affect the performance speed of the flash memory.
A second factor which also adversely affects the performance speed of a flash memory is the availability of an adequate power supply during write operations. Flash memories include arrays of flash cells, or transistors, that are electrically programmed using relatively high drain and gate bias voltages. However, designers of electrical systems that incorporate flash memories prefer not to include high voltage supplies in the electrical systems in order to reduce electrical system cost and power consumption. As a result, flash memories are designed for use with low voltage supplies having voltages, for example, as low as three volts.
Flash write operations required a relatively substantial current. For example, it is common to use 500 microamps per memory cell during a right operation. A two-byte write operation, therefore, requires 8 milliamps of current at a six-volt power supply. This power is not a major concern where an external Vpp supply is available. However, in low voltage memory devices the availability of this external supply is often eliminated. Thus, the flash memories are also designed to include charge pumps to create the required higher Vpp supply voltages. In addition, internal Vpp supply generators or charge pumps are limited, primarily due to economic reasons, in the amount of current which can be provided during write operations.
Conventional charge pumps can simultaneously program only relatively few flash cells. The number of memory cells, therefore, written during one write cycle is reduced into segments. One typical 16-bit programming operation is performed 4 bits, rather than 16 bits, at a time. This segmented programming operation significantly increases the programming time of flash memories. For example, a flash memory which is specified to operate a three volt power supply may be limited to writing 4 bits of data at once. To write two bytes of data, four separate write operations are required. Similarly, a flash memory which is specified to operate at six volts may be limited to writing one byte of data at once. To further complicate operating speed during write operations, as explained above, rewrite operations are often necessary.
Therefore, there is a need to reduce the programming time of flash memories. For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a flash memory device which can operate using a low voltage supply and an internal Vpp voltage generator during write operations, while minimizing time requirements for writing data to memory cells.